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Four Year Old libssh Bug Leaves Servers Wide Open

A fairly serious 4-year old libssh bug has left servers vulnerable to remote compromise, fortunately, the attack surface isn’t that big as neither OpenSSH or the GitHub implementation are affected.

The bug is in the not so widely used libSSH library, not to be confused with libssh2 or OpenSSH – which are very widely used.

There’s a four-year-old bug in the Secure Shell implementation known as libssh that makes it trivial for just about anyone to gain unfettered administrative control of a vulnerable server. While the authentication-bypass flaw represents a major security hole that should be patched immediately, it wasn’t immediately clear what sites or devices were vulnerable since neither the widely used OpenSSH nor Github’s implementation of libssh was affected.

The vulnerability, which was introduced in libssh version 0.6 released in 2014, makes it possible to log in by presenting a server with a SSH2_MSG_USERAUTH_SUCCESS message rather than the SSH2_MSG_USERAUTH_REQUEST message the server was expecting, according to an advisory published Tuesday.

The issue is to do with the fact the libssh server uses the same state machine to authenticate both clients and servers, the message dispatching code processes these requests with the same function and it doesn’t very which mode it’s running in.

This libssh bug is a case of clever exploration and understanding how a protocol works rather than fuzzing or brute-forcing to find a pretty serious vulnerability.

On the brighter side, there were no immediate signs of any big-name sites being bitten by the bug, which is indexed as CVE-2018-10933. While Github uses libssh, the site officials said on Twitter that “GitHub.com and GitHub Enterprise are unaffected by CVE-2018-10933 due to how we use the library.” In a follow-up tweet, GitHub security officials said they use a customized version of libssh that implements an authentication mechanism separate from the one provided by the library. Out of an abundance of caution, GitHub has installed a patch released with Tuesday’s advisory.

Another limitation: only vulnerable versions of libssh running in server mode are vulnerable, while the client mode is unaffected. Peter Winter-Smith, a researcher at security firm NCC who discovered the bug and privately reported it to libssh developers, told Ars the vulnerability is the result of libssh using the same machine state to authenticate clients and servers. Because exploits involve behavior that’s safe in the client but unsafe in the server context, only servers are affected.

A search on Shodan for this libssh bug shows about 6300+ sites using libssh, which isn’t exhaustive and also just the existence of libssh (like the GitHub implenmentation) doesn’t make it vulnerable.